The long-term objective is to determine the role of respiratory tract cytochrome P450 (P450 or CYP) enzymes in target tissue metabolic activation and toxicity of environmental chemicals. Our focus continues to be on CYP2A13, an enzyme selectively expressed in human respiratory tract, and the most efficient human P450 enzyme in the metabolic activation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a major tobacco- derived respiratory tract procarcinogen. CYP2A13 is also known to metabolize numerous other important respiratory tract toxicants. Our hypothesis, that CYP2A13 plays an important role in tobacco-related lung carcinogenesis in humans, is supported by findings of a recent epidemiological study, and by reports confirming that CYP2A13 protein is expressed in human lung, where it is active in the metabolic activation of NNK, and that P450s in the lung, but not those in the liver, are essential for NNK-induced lung tumorigenesis in mouse models. Furthermore, our preliminary finding, that expression of CYP2A13 is down-regulated by inflammation, offers an explanation for why the levels of CYP2A13 protein detected in patient-derived lung biopsy samples were so low, and suggests the possibility that CYP2A13 levels in intact, healthy lungs are much higher. Here, we propose three series of experiments to overcome the difficulties associated with not being able to directly study P450 expression or activity in intact, healthy human lungs. We will 1) study a CYP2A13-humanized mouse model, in order to provide proof-of-principle for the potential of CYP2A13 to mediate NNK-induced lung tumorigenesis in humans; 2) perform additional studies to better understand the nature and scope of inflammation-induced suppression of CYP gene expression in the lung; and 3) identify common CYP2A13 genetic variants that cause changes in gene expression (and the underlying mechanisms), in order to provide biological basis for future epidemiological studies aimed at further confirming the role of CYP2A13 in smoking-induced lung cancer or other chemical toxicities in various ethnic or occupational groups. We believe that our proposed studies are novel, and the anticipated outcome will be highly relevant to mechanisms of chemical carcinogenesis and other chemical toxicities in human lung.